The State Ports Authority is bracing for a possible waterfront strike by extending operations at its busiest terminals to lessen the impact of a walkout by the longshoremen’s union.

In an email sent to shipping lines Thursday, the SPA detailed steps to lessen the impact of cargo flow, including two extra operations hours in the weekdays following Christmas.

The agency’s two main container docks in Mount Pleasant and North Charleston also will be open for business on Saturday, Dec. 29, a day usually reserved for truck traffic only.

SPA spokeswoman Allison Skipper said the extended hours are designed to give shippers more time to move more of their cargo across the state’s ports.

Talks between shipping companies and dockworkers broke down Tuesday, less than two weeks before the contract expires Dec. 29, leading to worries that a strike was inevitable.

The National Retail Federation wrote to President Barack Obama this week to ask him to use “all means necessary” to head off a walkout, which they fear could have catastrophic ripple effects nationwide.

“We foresee this as a national economic emergency,” said Jonathan Gold, the group’s vice president of supply chain and customs policy.

Gold said billions of dollars in commerce at businesses nationwide could be affected, from auto manufacturers awaiting parts to the truckers that deliver them.

A strike wouldn’t affect passenger cruise ships, U.S. mail, military cargo or perishable cargo with a limited shelf life. It also wouldn’t affect cars and other non-containerized cargo known as break bulk.

The International Longshoremen’s Association union represents 14,500 workers at the Port of Charleston and 14 others that extend south from Boston. They handle 95 percent of all containerized shipments from Maine to Texas.

The impasse comes during a 90-day extension of the current contract. On Tuesday, a federal mediator offered another monthlong extension. Various issues, including wages, are unresolved, but the sides couldn’t agree on what’s become the key sticking point, container royalties.

The royalties are payments to union workers based on the weight of cargo received at each port. They were created in the 1960s to boost wages and finance worker benefits after increased automation cut into salaries and jobs, making it impossible for the dwindling labor force to finance its benefits, said ILA spokesman James McNamara.

The container carriers and port operators, represented by the U.S. Marine Alliance, want to cap the royalties at 2011 levels, saying they have morphed into a huge expense and a totally unrelated to their original purpose, which hurts the industry’s competitiveness as it tries to keep up with new technology.

The alliance said the royalty payments now amount to a bonus averaging $15,500 annually for East Coast workers who already earn more than $50 per hour.

The Associated Press and Tyrone Richardson of The Post and Courier contributed to this story.